This invention refers to double-acting pneumatic cylinders, and in particular concerns improvements to pneumatic cylinders of compact or short stroke type.
Pneumatic cylinders of compact or short stroke type are generally used for small or medium-powered actuators; they substantially comprise a tubular body and two front and rear heads which together define a piston chamber in which a piston member reciprocates.
Both the front and the rear heads of a pneumatic cylinder are provided with inlet-outlet ports for feeding and discharging pressurized air, which open out into a central bore or cavity in the head communicating with the piston chamber; a metal sleeve is normally provided in the front head to guide a piston rod during the reciprocating movement of the piston member.
Pneumatic cylinders of compact or short-stroke type are illustrated, for example, in DE-A-40 41 992, WO-A-94/00706 and EP-A-0 692 639.
As can be seen from these documents, in particular from EP-A-0 692 639 which relates to the closest prior art, the side heads for closing the piston chamber generally consist of end plates having a limited thickness in order to maintain the lengthwise dimensions of the cylinder within values defined by specific standards; the tubular body of the cylinder or the heads are provided with inlet-outlet ports for the pressurized air, which open out directly into the piston chamber or into a cavity in the closing heads.
Due to their limited dimensions, in particular due to the reduced thickness of the side heads, until now it has been difficult, or even impossible, to provide compact cylinders with suitable pneumatic devices for cushioning and controlling the speed of the piston member at one or both ends of its power stroke, for example of the type shown in U.S. Pat. No. 3,440,930, U.S. Pat. No. 3,805,672, EP-A-0 005 407.
As can be seen from these documents, a pneumatic cushioning device, for pneumatic cylinders of conventional type, normally comprises a venting duct for venting the pressurized air remaining in the chamber of the cylinder, along the final portion of the piston stroke; the venting duct comprises a narrow passage or a throttle valve, made suitably adjustable, to control the outflow of the air and, consequently, control the speed of the piston along said final portion of its power stroke. The cushioning device also comprises a plug or closing member for closing the air inlet-outlet ports, normally consisting of a cone-shaped or cylindrical plug element which axially extends in respect to the piston rod, and is designed to penetrate into a central bore or cavity of the head, to seal against a peripheral gasket; in this way the pressurized air is allowed to flow from the piston chamber exclusively through the venting duct of the pneumatic cushioning device.
However, as can be seen in the aforementioned documents, in a conventional pneumatic cylinder the central cavity into which the inlet-outlet ports for the pressurized air open out, and the closing cone or plug member in correspondence with the front head, are axially aligned with the guide bush for the piston rod.
Consequently, the presence of the cushioning device for controlling the piston speed in pneumatic cylinders of conventional type necessarily calls for heads of considerable width, capable of containing within their thickness the aligned disposition of the central air inlet-outlet cavity and the guide bush for guiding the piston rod.
For various reasons it has never been possible to fit usual pneumatic cushioning devices on compact cylinders due to the limited dimensions and thickness of the closing heads, except by excessively increasing the overall lengthwise dimensions of the cylinder, in respect to standardized sizes.
The main object of this invention is to provide a pneumatic cylinder of compact type, provided with a pneumatic device for cushioning and controlling the speed of the piston at the end of its power stroke, of such kind as not to involve any substantial increase in length of the cylinder, maintaining its overall dimensions within standards and within dimensions normally adopted.
A further object of this invention is to provide a pneumatic cylinder of compact type provided with an extremely simple and highly efficient pneumatic cushioning device, while maintaining the overall dimensions of the cylinder comparatively smaller than those of a conventional cylinder.
For example, in a conventional ISO cylinder having a piston chamber with a diameter of 50 mm, in general the cushioning stroke require approximately 60 mm in length.
Conversely, in a compact cylinder according to the invention, also having a piston chamber of 50 mm in length, it is possible to obtain a cushioning stroke, for example ranging from 15 to 35 mm without increasing the overall dimensions, and in any case obtaining an effective braking action and control of the piston speed.
In particular, the invention is directed to a cylinder of compact type, comprising:
a tubular body, defining a piston chamber;
front and rear heads for closing the piston chamber at both ends; each closing head for the piston chamber in turn comprising a pressurized air inlet-outlet port which opens out towards a central bore coaxially arranged and communicating with the piston chamber inside the tubular body;
a reciprocable piston member in said piston chamber, provided with a piston rod axially extending through a guide bush in the front head; and
pneumatic cushioning means for controlling the movement of the piston member at least one end of its stroke, said cushioning means comprising a venting duct in the closing head for venting air from the piston chamber, and a plug member which axially extends along a side end of the piston member to sealingly penetrate into the central bore of said head to tightly close the piston chamber towards the air inlet-outlet port, wherein:
the air inlet-outlet port of the front head opens into an annular slot coaxially extending into the same front head and longitudinally overlapping for at least part of the guide bush for the piston rod;
in that the plug member comprises a sleeve member coaxially extending from a side end of the piston member, and along the piston rod, said sleeve member being disposed to penetrate into the annular slot to overlap the guide bush; and
sealing means at the open end of the annular slot facing the piston chamber, said sealing means being provided to sealing close against the sleeve member when threaded into said annular slot.
According to a first embodiment, the annular slot is delimited by the outer cylindrical surface of the guide bush for the piston rod, and by the inner cylindrical surface of the central bore in the closing head.
According to a further embodiment, the annular slot in provided directly in the head in a position encircling the guide bush for the piston rod.